1. Field of the Invention
This invention relates to filament-wound fiber-resin composites having advanced structural properties and uncured resin mixtures used in their production.
2. Prior Art and Statement of Needs
Solid propellant rocket motor cases for missile systems, spacecraft boosters, and other types of large and small high-performance, lightweight pressure vessels are presently made from fiber reinforcement and various formulations of polyepoxide (epoxy) resins by the filament winding process. Similarly, filament winding with both polyesters and epoxy resins has made possible production of lightweight tanks, poles and piping. Historically, fiberglass has been the most common reinforcement fiber but other fibers such as carbon (graphite) filaments, boron filaments, and high modulus organic polymer filaments, most significantly aramid filaments, have been increasingly employed to take advantage of their differing physical properties.
The resins utilized are typically epoxy formulations based on diglycidyl ether bis-phenol A (DGEBA), reactive low-molecular weight epoxy diluents and curing agents consisting of aliphatic and aromatic amines and carboxylic acid anhydrides. Both flexibilized and rigid epoxy resins have been successfully used as matrices for the filament-wound composite structures.
The epoxy-based resins have not been without their disadvantages in this application. They tend to be somewhat high in density. For example, densities of 1.2 to 1.3 g/cc are often seen. Also, they generally require relatively high proportions of resin, say 35+% by weight, in the final composite. Composites based on epoxy resins are somewhat polar and hydrophilic and thus, with time, pick up weight via absorption of moisture from the atmosphere. These three problems can be deleterious in aerospace applications where low weight is a most sought after property. In addition, when epoxy-based resins are used with high modulus organic fibers, such as the aramid fibers, there is often enhanced crack propagation due to weakness in the fiber's radial dimension and thus strength reduction. Classically this strength reduction is corrected by precoating the fibers with substantial amounts of low viscosity lubricating or release agents. This prevents a too tight bond between the epoxy and fiber and distributes loads more evenly through the aramid fiber windings. This solution has a concurrent problem as the additional release agent adds weight to the composite.
A need exists for an improved resin-fiber composite which is of low density and low water pick-up and that, through proper fiber-resin interactions, gives high strength. The desired resin system should have physical and chemical properties in its uncured state, such as low viscosity and good pot life, that are conducive to successful use in production scale filament winding.
It is an object of this invention to provide resins and resin-fiber composites that meet these needs.